1. Field of the Invention
This invention relates generally to fixed cutter rotary drag bits for earth boring and, more particularly, to improvements in bit design for so-called "anti-whirl" bits.
2. State of the Art
Fixed cutter rotary drag bits for subterranean earth boring have been employed for decades. It has been found that increasing the rotational speed of such drill bit has, for a given weight on bit, increased the rate of penetration of the drill string. However, increased rotational speed also has tended to decrease the life of the drill bit due to premature damage to and destruction of cutting elements, commonly polycrystalline diamond compacts (PDC's).
It has recently been recognized that cutting element destruction, particularly at higher rotational speeds, is at least in part attributable to a phenomenon known as "whirl" or "bit whirl." Radially directed centrifugal imbalance forces exist to some extent in every rotating drill bit and drill string. Such forces are in part attributable to mass imbalance and in part to dynamic forces generated by contact of the drill bit with the formation. In the latter instance, aggressive cutter placement and orientation creates a high tangential cutting force relative to the normal force and aggravates the imbalance. In any event, these imbalance forces tend to cause the drill bit to rotate or roll about the bore hole in a direction counter to the normal direction of rotation imparted to the bit during drilling. This counter-rotation is termed "whirl," and is a self-propagating phenomenon, as the side forces on the bit cause its center of rotation to shift to one side, after which there is an immediate tendency to shift again. Since cutting elements are designed to cut and to resist impact received in the normal direction of bit rotation (clockwise, looking down), contact of the cutting elements with the bore hole wall in a counter-clockwise direction due to whirl places stresses on the cutting elements for which they were never designed.
One solution to the problems caused by bit whirl has been to focus or direct the imbalance forces as a resultant side force vector to a particular side of the bit via changes in cutting element placement and orientation and bit mass location, and to cause the bit to ride on a low-friction bearing zone or pad on the gage of that side of the bit, thus substantially reducing the drill bit/bore hole wall tangential forces which induce whirl. This solution is disclosed in numerous permutations and variations in U.S. Pat. Nos. 4,982,802; 4,932,484; 5,010,789 and 5,042,596, all assigned to Amoco Corporation of Chicago, Ill.
The above-referenced patents generally require that the low friction bearing zone or pad on the gage and adjacent bit profile or flank be devoid of cutters, and indeed many alternative bearing zone configurations are disclosed, including wear coatings, diamond stud inserts, diamond pads, rollers, caged ball bearings, etc. It has been suggested that the bearing zone on the bit gage may include cutting elements of different sizes, configurations, depths of cut and/or rake angles than the cutters located in the cutting zone of the bit, which extends over the bit face from the center thereof outwardly to the gage, except in the flank area of the face adjacent the bearing zone. However, it is represented in the prior art that such bearing zone cutters should generate lesser cutting forces than the cutters in the cutting zone of the bit so that the bearing zone will have a relatively lower coefficient of friction. See U.S. Pat. No. 4,982,802, Col. 5, lines 29-36; U.S. Pat. No. 5,042,596, Col. 4, lines 18-25.
While anti-whirl bits have been built according to the aforementioned designs, the use of a cutter-devoid bearing zone and adjacent profile has resulted in excessive wear of the bearing zone as well as of the cutters on the flank of the bit, which shortens bit life even when cutting elements still have significant life remaining. This problem manifests itself most dramatically when the bit has to ream to reach the bottom of the hole.
Therefore, in order to take full advantage of the anti-whirl bit concept, it would be desirable to possess an anti-whirl drill bit having cutters placed on the bit profile adjacent the bearing zone of the bit in such a manner that the reaming capabilities and wear resistance of the bit to high side loads is enhanced without adversely affecting the anti-whirl tendencies of the bit.